An Overview of 3-D Geometric Models to Describe Brittle Fracture in Polycrystalline Ferritic Steels

Flaws that can be present within pressure vessels, pipework and other engineering components are assessed using the principles of fracture mechanics. In general, failure avoidance methodologies are used for assessments. In these, mechanical properties and fracture toughness of the particular materials, taking account of the thermo-mechanical history, are important input parameters. In addition, however, it is necessary to support these approaches with an understanding of the underlying fracture mechanisms. In the case of lower shelf brittle fracture of ferritic materials, various approaches have been adopted to provide an understanding of the initiation and growth of these cracks. Certainly polycrystalline ferritic steels are widely used for construction of engineering components. Although a truly multiscale problem, this overview describes the key features of 3D geometric models that can be used to consider brittle fracture of a variety of polycrystalline materials, including bcc materials, in the intermediate microscale range of 10⁻⁶ to 10⁻³ m. Results of the modelling applied to the brittle fracture of bcc polycrystalline ferritic materials are presented, particularly with respect to predicted crack paths as an initiated crack propagates across the model. In this situation, brittle cleavage and intergranular brittle fracture are addressed. One particular application is the effect of prior grain boundary creep damage, arising from plant service history, on the subsequent lower shelf brittle fracture of ferritic steel components. The implications of this 3D modelling of polycrystalline material are discussed.